The present invention relates to a pneumatic cylinder device having a locking mechanims which locks the piston rod of the cylinder device during stroking as desired or in the event of a failure in the air supply.
Pneumatic cylinder devices having locking mechanisms are known. FIG. 4 shows, by way of example, a fluid-pressure type locking mechanims incorporated in some of these known pneumatic cylinder devices. This locking mechanism has a hydraulic pressure generating means 103 operatively associated with a brake 102 provided in the pneumatic cylinder 101. The force of a spring 104 incorporated in the hydraulic pressure generating means 103 is converted into pressure of a hydraulic fluid 105, which in turn is applied to the outer periphery of a brake metal 106 in the brake 102. As a result, the brake metal 106 having a slit 106' is radialy contracted so that the piston rod 107 is braked and stopped due to friction between the brake metal 106 and the piston rod 107.
FIG. 5 shows another known locking mechanism which may be referred to as "mechanical type" locking mechanism. This locking mechanism has a tightening member 114 having a wedging function and attached to a lock piston 113 resiliently mounted through a spring 112 in a lock cylinder 111 provided on the pneumatic cylinder 110. As the tightening member 114 is actuated, it presses a plurality of balls 115 which in turn press an annular lock shoe 117 on the peripheray of the piston rod 116 thus locking the piston rod 116.
The fluid-pressure type locking mechanism is advantageous in that the piston rod 107 can be braked by pressure which acts uniformly on the entire periphery of the brake metal 106, and in that it has an excellent durability because of small wear of the brake metal 106. However, this locking mechanism suffers from disadvantages in that the hydraulic pressure generating mechanism 103 projects laterally from the pneumatic cylinder 101 resulting in an extremely large size of the pneumatic cylinder device and in that troubles are often caused due to leak of the hydraulic fluid for operating the mechanism.
On the other hand, the mechanical-type locking mechanism involves various drawbacks, although it can provide a compact construction of the pneumatic cylinder device as a whole. Namely, in this locking mechanism, the braking pressure cannot be applied uniformly over the entire portions of the lock shoe or the brake metal 117 unlike the fluid-pressure type mechanism. In consequence, the lock shoe 117 makes contact with the piston rod 9 only at its selected portions so that it is worn locally resulting in a shorter life, i.e., inferior durability. In addition, the release of the wedging function of the tightening member 114 at the time of unlocking often involves a certain delay. Since different pneumatic pressures exist in the pressure chambers 119,120 at both sides of the piston 118 during locking, the delay in release of the wedging function may cause an increase of the pressure differential across the piston, resulting in an erroneous operation of the pneumatic cylinder device. Furthermore, the mechanical-type locking mechanism inherently suffers from a reduction in the efficiency due to friction in various mechancal parts.